FIGS. 22A–22C show the antenna disclosed in Japanese Patent Laid-Open Application No. 2002-232227. In a case where the antenna has a bandwidth of 100 MHz with a center frequency of 2450 MHz, a dielectric substrate with a dielectric constant of 8 is processed to have a size of 26 mm×26 mm and a thickness of 6 mm. Then, a patch electrode (hereinafter referred to as patch) 101 of 20 mm×20 mm is formed on the surface of the substrate so as to complete antenna element 100. The midpoints of two opposing sides of patch 101 are connected with each other, and the midpoints of the other two opposing sides of patch 101 are connected with each other so as to form two lines intersecting at right angles. On these two lines, one power feed pin 102 is inserted in each of the two 50 Ω points (not an edge of the patch, but inside the patch), thereby resulting in two independent microstrip antennas whose polarization axes in the directions X and Y are orthogonal to each other. Wiring board 103 has a ground pattern on one entire side thereof except for a nonconductive region provided for power feed pins 102 of antenna element 100, and the ground pattern is the ground conductor of antenna element 100. Electric power is fed by power feed terminal 106 via hybrid circuit 105, and connection with an external circuit is performed via coaxial line 104. This structure can achieve a circularly polarized antenna with excellent axial ratio characteristics in a broad frequency range.
A problem of this conventional antenna is a complicated fabrication process. Specifically, the antenna has the power feed point not at an edge of the patch, but inside the patch, so that power feed pin 102 has to penetrate the dielectric member, thereby complicating the fabrication process.
Furthermore, the conventional antenna can radiate circular polarization only towards the top side of the ground pattern on which the patch antenna is mounted, and it is impossible to transmit signals towards the bottom side of the ground pattern. Providing directivity towards the bottom side requires disposing a microstrip antenna also on the bottom side of the ground pattern, and this leads to the problem of an increase in the cost and size of the antenna.
In addition, conventional antenna element 100 is made of a conductive pattern formed on the surface of wiring board 103 that has no components mounted thereon. If a patch antenna is disposed on wiring board 103 in order to have the directivity towards the bottom side, then no room is left for hybrid circuit 105. Consequently, a total of two hybrid circuits 105 have to be formed in a layer in wiring board 103, thereby further complicating the antenna structure and making antenna designing extremely difficult.